1. Field of the Invention
This invention relates to electrostatic proofing systems and, more particularly, to a method by which dot gain may be controlled to provide a preview of press results.
2. Description of the Prior Art
Reproduction of color images through printing is an involved process requiring a breakdown of an original image into a number of fundamental, single color images and the eventual recombination of the single color images through multiple printings into a full color reproduction of the original. In its most elementary form, color image reproduction requires the following sequential steps.
First, using filtering and other photographic techniques a number of color separation transparencies are produced (typically four) each bearing a halftone dot image corresponding to the magenta, cyan, yellow and black portion of the original, respectively. A transparency bearing a halftone dot image is one in which various optical densities of a given image are reproduced by a multiplicity of opaque or transparent dots per unit area, the dots having varying diameters.
Second, the color separation transparencies are next used to produce printing plates corresponding to each of the four colors.
Third, the printing plates are mounted on multistage printing presses which print sequentially, in register, one on top of another, four halftone images corresponding to each of the color separations to create a full color reproduction of the original.
The various colors and hues are reproduced by the superposition of a multiplicity of dots of varying diameters and colors corresponding to the four color separations. Any deviation in the size of the printed dots results in color shifts in the final printed image. As a result of this reproduction process, setting up the press and printing a four color image is economically feasible typically only when employed for printing large quantities of a given subject.
It is desirable to be able to predict the final image appearance before it is printed, and preferably before even making printing plates by studying images made directly from the color separation transparencies.
The art of evaluating the color separation transparencies and deciding whether the various colors have indeed been properly separated is called proofing. Proofing is a predictive process which uses the information in the color separations to create an image taking into account known effects or errors that normally occur during the printing process. Proofing is used to visualize what the final printed image will look like without actually making printing plates and running the printing press. This way, the color separations may be timely corrected.
Methods have been developed and are well known in the art to test the color separations without an actual printing run. See for instance "Principles of Color Proofing" by M. H. Bruno, published by Gama Communications, Copyright 1986.
One of the problems that affects the final print is variations in the diameter of the dots comprising the halftone image. For instance, assume that a given tint and color density in a portion of an image is to be reproduced accurately using a given set of inks and a combination of four color separation transparencies. These may be, for example, a yellow color separation with dots having a diameter of 0.010 inches, a cyan color separation with dots having a diameter of 0.020 inches, a magenta with dots of 0.030 inches and a black with dot diameters of 0.015 inches. Upon printing, however, the image shows an unacceptable tint. Upon measuring the actual printed dot diameters on the printed image, we find that even though the dots on the printing plates correspond exactly to the dot size on the color separations, the yellow dot diameter is smaller than 0.010 by 10% while the others are larger by 5% to 20%. This may have resulted because our printing press happens to be printing light on the yellow, but heavy on the other colors, resulting in dot shrinkage or growth. To compensate for that, we must make our printing plates with bigger diameter dots than the "correct" size for the yellow separation, and the others with smaller diameter dots than the "correct" size.
In order to have a quantitative, as well as a qualitative, understanding of this dot growth problem, and to be able to generate color separations that will eventually produce printing plates which compensate for the various changes to the dots in each particular press, there have been developed in the art measuring systems, such as the System Brunner Isocontour curves discussed in the aforementioned Bruno publication, in Pages 260 to 270. Using special printing test targets containing colored patches of preselected dot sizes, printed alongside an image, a press operator may measure colored areas having dots of a preselected diameter and determine any deviation therefrom. This in turn tells him how much dot growth or shrinkage is introduced by his press. By having this knowledge, and given a proofing system in which he can introduce at will dot growth or shrinkage similar to that of his press, he can produce proofs that show the exact output of his press, and can, therefore, correct his color separations accordingly.
While System Brunner Isocontour curves and the like, are very useful in predicting the results of press dot growth or shrinkage in traditional printing, they have not heretofore been used with electrostatic printers even though such printers are capable of producing four color prints in short runs and short times, such as described in U.S. patent application Ser. No. 07/117,269, filed Nov. 4, 1987, which issued into U.S. Pat. No. 4,849,784 and assigned to the assignee of the present invention.
What is needed is a method to produce color proofs in electrostatic printing equipment and to allow one to evaluate the color separations, without having to make printing plates and prints on a printing press.
Therefore, it is an object of this invention to provide a method which allows one to electrostatically produce color proofs that accurately predict the results of actual press runs for a given press of known characteristic response, i.e., whose dot gain versus the dot percentage coverage is known or can be measured.